In modern society, batteries are widely used in portable electronic products such as notebooks, computers, cameras, cellular phones and MP3 players, and various devices such as automobiles, robots, and satellites. Batteries may be classified into primary batteries and secondary batteries, and the secondary batteries are widely used because they have great advantages in terms of energy storage as well as repeated charging and discharging.
Secondary batteries commercially available at the present include nickel-cadmium batteries, nickel-hydrogen batteries, nickel-zinc batteries, lithium secondary batteries and the like. Among them, lithium secondary batteries are attracting attention due to very low self-discharging rate, high energy density, and free charging and discharging, caused by substantially no memory effect, in comparison to nickel-based secondary batteries.
In the secondary battery, a single battery cell may be used alone, but two or more battery cells may be electrically connected to each other in a single battery system so as to output or store higher power. In particular, as carbon energy is getting depleted and environmental concern is increasing recently, a battery system using a secondary battery is increasingly used in large-sized equipment. Since the battery system used in the large-sized equipment requires high output or high capacity, a large number of battery cells may be used in an electrically connected state.
A smart grid system is an example where a large battery system including a plurality of battery cells electrically connected as described above is used.
The smart grid system is an intelligent power network system for enhancing the efficiency of power utilization through the interaction of power supply and consumption by integrating the information and communication technology into power production, transportation and consumption processes. The amount of power consumed by a consumer is not always constant and may vary from time to time. Representatively, in the afternoon of summer, the amount of used power may abruptly increases due to the use of an air conditioner, and at night, the amount of used power decreases rapidly. In this way, the power consumption may not be consistent but frequently fluctuate in terms of a power consumer, but it is practically difficult for a power supplier to meet the changing amount of power consumption even though the power generation amount is controlled to some extent. Thus, power supply overload or power supply shortage may occur due to the unbalance of power supply and consumption. In order to solve this problem, the smart grid system checks a power usage state in real time and flexibly adjusts the power supply amount. One of important components for the smart grid system is a battery system for power storage. In the battery system for power storage, a much larger number of battery cells are electrically connected, in comparison to a battery system used for general electronic equipment or portable electronic products.
In addition, the power storage device in which many battery cells are electrically connected to each other may be used not only in the smart grid system but also in other fields such as a micro grid system or an electric vehicle charging station which supplies charge power to electric vehicles.
In the case of a battery used for the power storage device, the power consumption or the power supply is irregular, and thus the charge power or discharge power is not constant and frequently varies with time. In such a situation, if the configuration of an existing battery system or an existing battery management technology is applied to a power storage device or the like without any change, a power supply situation changing in real time may not be efficiently handled. In particular, if a specific battery cell is used excessively in comparison to other battery cells, the lifetime of the corresponding battery cell may be significantly reduced or have an increased risk of failure. Thus, in this case, the performance of the power storage device or the like may be degraded or the possibility of failure may increase. Therefore, in a battery system including a plurality of battery cells, it is very important how to use each battery cell.